As a next-generation system for performing high speed communication with high capacity, as compared with the 3rd-generation and 3.5th-generation cellular radio communication systems operated at present, there are LTE (Long Term Evolution), which is standardized by 3GPP (3rd Generation Partnership Project) of a standardization body, and LTE Advanced which is a sophisticated version of LTE.
In a downlink of an LTE system (including LTE Advanced), a base station transmits user data to a radio terminal using a data transmission channel called PDSCH (Physical Downlink Shared Channel). In addition, the downlink indicates communication toward the radio terminal from the base station, and an uplink indicates communication toward the base station from the radio terminal.
Furthermore, in LTE Advanced, it is discussed to provide a heterogeneous network, that is a network in which a low power base station (so-called a pico cell base station, a femto cell base station, or a relay node) is located in a communication area of a high power base station (so-called a macro cell base station). In the heterogeneous network, it is possible to distribute a load of the high power base station to the low power base station.
However, since it is general that a radio terminal is connected to a base station, which has the highest received power of a radio signal among a plurality of base stations, it is probable that a connection opportunity of the radio terminal to a low power base station with low transmission power is reduced in the heterogeneous network.
In this regard, there has been proposed a technique of expanding a coverage (a communication area range) of a low power base station by controlling a radio terminal to be connected to the low power base station even though received power from the low power base station is not highest (for example, refer to Non Patent Literature 1).